Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become very popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image. The clear coat is particularly important for these properties.
Coating compositions based on hydroxyl functional acrylic polymers and aminoplast crosslinking agents have been particularly desirable as automotive top coats because they exhibit excellent durability, hardness, gloss and appearance. Conventional coating systems that contain hydroxyl functional film-forming resins and aminoplast crosslinking agents rely on a cure mechanism wherein hydroxyl groups on the resin react with the aminoplast to form ether linkages. See, for example, European Patent Application 0 257 848. Historically, such coating systems have been vulnerable to acid attack and hence show poor acid etch resistance.
Because many geographic areas encounter acidic precipitation, acid resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings. When acidic rain water, also known as acid rain, accumulates on a horizontal section of an automobile such as a hood or trunk deck, it spots and can actually etch away the clear coat leaving a blemish that cannot be polished out. Hydroxyl-aminoplast coating systems of the prior art are not highly effective for providing protection against etching caused by acid rain.
Alternative curing systems can be used which provide improved acid etch resistance such as hydroxyl-isocyanate, epoxy-acid and carbamate-aminoplast; however, these systems are generally more costly than the hydroxyl-aminoplast coating systems, and in certain instances, lack some beneficial properties of hydroxy-aminoplast systems such as mar resistance or hardness.
It is an object of the present invention to provide a coating composition having improved acid etch resistance, utilizing inexpensive hydroxyl-aminoplast curing for use in a color-plus-clear composite coating system.
Surprisingly, it has been found that certain addition polymers derived from beta-hydroxy ester functional monomers can be cured with aminoplast crosslinking agents to yield acid etch resistant coatings.